For example, like a CD and a DVD, an optical disc for optically recording or reproducing data using a laser beam or the like has been developed. In such an optical disc, there may be variation in substrate thickness. The variation in substrate thickness leads to the occurrence of spherical aberration. Therefore, in order to perform preferable recording and reproducing operations, it is necessary to reduce the influence of the spherical aberration caused by the variation in substrate thickness. One method developed for this purpose is a method of focusing the laser beam on a recording surface in accordance with the variation in substance thickness by changing the position of a collimator lens (or a condenser lens) in the optical path of the laser beam. Namely, a method of focusing the laser beam on the recording surface in accordance with the substrate thickness by converting the magnification of an optical system is developed.
On the other hand, the following technology is introduced into such an optical disc: a technology of controlling the light intensity of the laser beam emitted from a laser diode, by monitoring the light intensity of the laser beam emitted from the laser diode and by performing feed-forward control or feedback control on the basis of the monitored light intensity. Here, a patent document 1 discloses an optical disc apparatus for monitoring the light quantity of the laser beam after the position of the collimator lens is changed. Specifically, by performing opening limitation using an aperture on a lens which constitutes the optical system for monitoring the light intensity, the numerical aperture of the lens which constitutes the optical system for monitoring the light intensity is matched with the numerical aperture of an objective lens, and the optical arrangement of the lens, which constitutes the optical system for monitoring the light intensity, with respect to the collimator lens is matched with the optical arrangement of the objective lens with respect to the collimator lens. By this, it is possible to control the light intensity of the laser beam emitted from the laser diode, in view of variation in the light intensity of the laser beam at the objective lens, which focuses the laser beam on the optical disc.
Patent document 1: Japanese Patent NO. 3538171